The present invention involves a refrigeration control system for controlling the operation of a refrigeration system in order to insure efficient operation of the system and that the refrigerant discharge from the evaporator is sufficiently superheated and above the saturation temperature so that it is completely converted into a gaseous state before being returned to the compressor.
In recent years, numerous technological advances have been made for optimizing the energy use of a refrigeration system by lowering the condensing temperatures and increasing the liquid subcooling such as disclosed in U.S. Pat. No. 4,286,437, entitled "Energy Saving Refrigeration System," and No. 4,304,100, entitled "Energy Saving Refrigeration System With Mechanical Subcooling." As a result of such advancements, the capacity of the compressors within the refrigeration systems have increased by as much as 40%. In addition, during mild load conditions that exist during the cold season, the load condition on the evaporator coil may drop by as much as 30%. Since the compressors are sized to handle operations at high load conditions, i.e. during the warmest time of the year, and the compressors may have a large excess of capacity under low load conditions. This often leads to a large fluctuation in the temperature of the display case. By improving the efficiency of operation of the system, the load is decreased and the size of the compressors can be decreased.
In order to help maintain proper operation of the evaporator coil and the compressors, it is mandatory that the refrigerant passing through the evaporator coil is fully converted into a gaseous state. If the refrigerant is not fully converted into a gaseous state, but remains at least partially liquified, this liquid when discharged into the compressor can result in severe damage to the compressor. In order to insure the complete conversion of the refrigerant from a liquid to a gaseous state within the evaporator, a sensing device has been placed at the outlet of the evaporator coil to maintain specific preset superheated conditions, normally from 3.degree. to 10.degree. F. This operation, however, is not based upon actual load conditions but only a comparison of the measured temperature with a preselected temperature. The comparison that is made then is used for controlling operation of the expansion valve so as to vary the flow of refrigerant into the evaporator in order to achieve the desired level of superheat as compared to the preset level of superheat.
Other types of controls have been employed for trying to avoid flooding of the compressor. One typical type of control that has been used includes a thermostat and liquid line solenoid which is connected to the input side of the evaporator coil. If the temperature measured by the thermostat arranged at the output side of the evaporator coil falls then the flow of liquid to the evaporator coil is turned off by control of the liquid line solenoid. This process will tend to result in pumping out the refrigerant from the evaporator coil when the solenoid blocks the flow of the refrigerant. When the temperature again rises, the solenoid valve is opened and the coil again will be flooded. This process often leads to high fluctuations in the temperature of the air passing over the evaporator coil.
Another type of control relies upon the use of evaporator pressure regulators. Such regulators are installed in the suction line of the compressor and are used to keep the suction pressure of the compressor constant all the time. Two problems with this type of regulator are: first, that it operates independently of the load conditions at the display case and therefore has a tendency to cause the display case to run several degrees colder in the mild seasons and several degrees warmer in the colder seasons and second, that in order for the evaporator pressure regulator to keep the same suction pressure at the evaporator coil it has to modulate between open and closed positions which creates a pressure drop typically of 3-7 psi. Typically every pressure drop of 2 psi in the system leads to a 5% greater consumption of energy by the compressor.
A third type of control system that has been utilized is a temperature pressure regulator. The problem incurred with the use of such a temperature pressure regulator relates to the pressure drop that is incurred during operation of the controls which results in increasing the energy consumption.
In order to improve the efficiency of operation of the refrigeration system, several different types of electronic control systems have been developed. In several of these systems, an electronically controlled expansion valve is utilized such as shown in U.S. Pat. No. 3,872,685 to Matthis. In such patent, the temperature at the outlet of the refrigeration air conduit is measured and such temperature is compared with a preselected temperature for then controlling the operation of the expansion valve. An expansion valve of the type shown in this patent is currently marketed by Singer Controls Company of America under the name Thermal-Electric Expansion Valve (R-205); in a brochure describing such expansion valve a temperature sensor, a thermistor, is shown inserted into the discharge line of the evaporator for providing a feedback signal for controlling operation of the Thermal Electric Expansion Valve.
Two other electronic control systems for refrigeration systems are disclosed in U.S. Pat. Nos. 4,102,150 and 4,112,703 to Kountz. Both of these patents disclose systems for controlling an air conditioning refrigeration system used in an automobile.
In the first of the patents to Kountz, the temperature along the discharge line from the evaporator coil is measured and the temperature within the actual space being air conditioned is measured and each of these temperatures is compared to a set temperature and in response to such comparison output signals are provided for controlling the operation of a solenoid operated control valve located in a by-pass line around the compressor for controlling the crank case pressure within the compressor. The first temperature sensor located in the space being air conditioned provides a space temperature signal that represents the actual ambient temperature. A circuit then provides a temperature set point signal representing the desired temperature for the space being air conditioned. A comparator responds to the space temperature signal and the temperature set point signal to produce a temperature control point signal which represents a desired evaporator refrigerant outlet temperature. The second temperature sensor positioned adjacent to the discharge line from the evaporator provides an evaporator outlet temperature signal representing the actual temperature of the refrigerant in such discharge line. An error signal is then provided, which error signal is responsive to the temperature control point signal and the evaporator outlet temperature signal. This error signal in turn is used for controlling the solenoid associated with the compressor for increasing the crankcase pressure of the compressor.
In the second of the two patents to Kountz, the temperature along the discharge line of the evaporator as well as the temperature within the actual space being air conditioned again are both measured. The processing of the temperature signals is similar to the processing in the above-noted first patent to Kountz. In the operation of the control system disclosed in this second patent to Kountz the objective is to maintain a substantially constant desired temperature in the space being air conditioned. The control system monitors both the ambient temperature in the space being air conditioned as well as the temperature of the refrigerant at the discharge line of the evaporator. An electromechanically controlled valve is included in series with the refrigeration circuit between the condenser and the evaporator and serves to control the flow of refrigerant through the evaporator. The control circuit responds to both of the temperature monitors for varying the actuation of the electromechanical valve to modulate the flow of refrigerant through the evaporator to adjust the refrigerant temperature at the evaporator outlet as required to maintain the controlled space at the desired temperature.
U.S. Pat. No. 3,807,192 to Porter describes a flow control system for varying the capacity of the refrigeration system. U.S. Pat. No. 4,129,995 to Usami discloses a refrigeration control system for controlling an expansion valve in response to the temperature and pressure of the refrigerant discharged out of the evaporator.